China has spent years building military outposts on a group of contested islands in the South China Sea — a project that has left the country at odds with many of its neighbors and the United States.

First, there was the dredging, in which ships sucked sediment from the seabed and pumped it atop formerly undeveloped reefs. Then came the buildings — once said to be for civilian purposes but which analysts now say are small military installations — followed quickly by international uproar.

But the building continued. Now, some of the islands that are part of the group known as the Spratlys, where China began large-scale development in 2013, have been transformed from barren reefs into military outposts, as seen for the first time in great detail in a series of new photos.

The images, which were obtained by The Philippine Daily Inquirer from an unnamed source, offer the clearest views yet of the scale of construction and the nature of the military development.

The Pentagon and the Philippine military both declined to comment on the images. The New York Times has been unable to independently verify these photos, which were released with annotations that indicate they were taken between June 2016 and December 2017.

But experts who monitor development in the South China Sea say the photos show the islands in dispute and are consistent with satellite imagery of the development that they have been monitoring for years.

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Philippine Daily Inquirer Photo

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Philippine Daily Inquirer Photo

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Philippine Daily Inquirer

Conor Cronin, a research associate at the Asia Maritime Transparency Initiative at the Center for Strategic and International Studies in Washington, said the buildings in the new images were at the same scale as the structures in the photographs his group regularly assesses from the region. The initiative monitors development in an area long shrouded in secrecy.

What’s striking about these images, he said, is that for the first time, significant surface-level details can be seen.

“These are kind of rare shots to see publicly,” Mr. Cronin said, noting that the detailed views of the island, seen from a much closer angle than the satellite level, were insightful. “It’s another indication of how well established these facilities are and kind of the capabilities they are going to have.”

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Philippine Daily Inquirer Photo

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Philippine Daily Inquirer Photo

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Philippine Daily Inquirer

The development of the Spratly Islands has ignited outrage from neighbors that also lay claim to this portion of the South China Sea. To varying degrees, the Philippines, Taiwan, Vietnam and Malaysia all stake claim to parts of the area.

While tensions have eased somewhat as China negotiates an agreement with other players on a code of conduct for the region — which seems to allow economic development without resolving the underlying sovereignty disputes — the unabated pace of construction could stir fresh tensions.

Bonnie S. Glaser, a senior adviser for Asia at the Center for Strategic and International Studies, said the photos provided a unique perspective. She said she believed, particularly given the annotations on the images, that they were most likely taken by a military aircraft.

“What is really quite stunning is how clear you can see in these aerial photos how much has been built,” Ms. Glaser said. “It’s really up close and personal.”

A recent assessment from the Asia Maritime Transparency Initiative of satellite imagery revealed that China had developed 72 acres, both above and below ground, in the South China Sea in 2017 alone. Since the construction first began in 2013, China has developed more than 3,200 acres across the area.

More recently, the Chinese have turned their attention from dredging and reclaiming land to building airstrips, radar and communications facilities and hangars.

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Philippine Daily Inquirer Photo

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Philippine Daily Inquirer Photo

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Philippine Daily Inquirer

On Mischief Reef, which was once largely underwater, 1,379 acres have been developed by China. The latest images from the island purportedly show a nearly two-mile runway and concrete building. Analysts say it is only a matter of time before Chinese military planes begin landing there.

Islands are colored by occupying country: China, the Philippines, Malaysia, Vietnam or Taiwan. Lines in the same colors show the extent of territorial claims.

China

South China Sea

Philippines

Claimed by the Philippines

China has long marked its claim with a “nine-dash line” that skirts the coasts of other countries.

Sulu Sea

Vietnam

100 Miles

Vietnam claims the Paracel and the Spratly Islands.

Claimed by Brunei

Malaysia

Brunei

Claimed by Malaysia

Indonesia

Islands are colored by occupying country: China, the Philippines, Malaysia, Vietnam or Taiwan. Lines in the same colors show the extent of territorial claims.

China

China has long marked its claim with a “nine-dash line” that skirts the coasts of other countries.

Philippines

Manila

Paracel Islands

Claimed by the Philippines

South China Sea

Vietnam

Fiery Cross Reef

Spratly Islands

Vietnam claims the Paracel and the Spratly Islands.

Malaysia

Brunei

Claimed by Malaysia

Hanoi

China

Manila

Vietnam claims the Paracel and the Spratly Islands.

Hainan

South China Sea

Paracel Islands

Philippines

Claimed by the Philippines

China has long marked its claim with a “nine-dash line” that skirts the coasts of other countries.

Visayas

Thailand

Palawan

Vietnam

Sulu Sea

Cambodia

Spratly Islands

Fiery Cross Reef

100 Miles

Ho Chi Minh City

Phnom Penh

Celebes Sea

Claimed by Brunei

Malaysia

Islands are colored by occupying country: China, the Philippines, Malaysia, Vietnam or Taiwan. Lines in the same colors show the extent of territorial claims.

Claimed by Malaysia

Brunei

Indonesia

Sulawesi

Claimed by Indonesia

Borneo

“They are going to begin to operate out of these runways and, I believe, start to actually store these aircraft, at least short term, in these hardened shelters,” Ms. Glaser said. “They built them for that purpose.”

China’s Ministry of Defense announced on Wednesday that it would conduct training exercises with combat jets in the South China Sea, though it did not indicate whether any of the planes were destined for the islands.